This invention relates generally to multi-layer interconnects for electrical circuits, and more particularly the invention relates to the interconnection of layers for circuit applications.
Multi-layer interconnects are used in electrical circuit fabrication for the interconnection of circuit components. Typically, the interconnect module comprises a ceramic such as alumina or other dielectric laminate material such as FR4, Getek, and BT, for example. Depending on module size, the module can have two metal layers on opposing module surfaces or multiple metal layers such as four positioned on opposing outer module surfaces and between dielectric layers within the module. The metal layers, aluminum or gold plated refractory metal for example, can be selectively patterned and etched for specific interconnect functions.
In a multi-layer metal module for microwave applications, the top layer is usually a signal strip line, the second layer is a ground plane, the third layer is a signal strip line, and the fourth or bottom layer is a ground layer. FIG. 1 illustrates the conventional structure for interconnecting the various layers through use of via holes filled or lined with solder or other suitable metal. The module includes metal layers 1, 2, 3, and 4 and dielectric layers 11, 12, 13 which separate the metal layers. Circuit components 15, 16, and 17 are selectively interconnected by the metal layers. For example, via 20 interconnects metal layers 1 and 2 for grounding, via 21 interconnects metal layers 1 and 3 for signal connection, via 22 interconnects layers 2 and 4 for common ground, via 23 connects layer 3 and layer 4 as a signal pad to the outside, and via 24 interconnects layers 2, 3, and 4 for grounding. It will be noted blind via holes are used which do not extend through the entire structure but only through dielectric layers necessary to access the interconnected metal layers. This use of blind via holes increases manufacturing steps and costs in fabricating the electronic modules.
In accordance with the present invention a multi-layer interconnect structure is provided in which all via holes extend through all layers of the module. Layers which are not to be interconnected by a via have a metal pattern in which metal is not present at the via location. Thus, via holes can be drilled through all layers of the module, and the vias will interconnect only the metal layers which have metal at the via location. Any metal layers in which the metal has been removed at the via location are not interconnected. Thus, blind via holes are not required, and this facilitates manufacturing processing and reduces costs.
The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawing.